Mito Castle was the seat of one of the three Tokugawa gosanke — the three cadet branches of the Tokugawa shogunate family that were considered the most prestigious lords in Japan, second only to the main Tokugawa line in Edo. Most of the castle complex was demolished in the Meiji period and rebuilt as a school, but the Ninomaru-saka gate (a rare surviving original structure), the earthen ramparts, and several moats remain.
The current Mito Daiichi High School occupies the inner citadel grounds and preserves the old gate. Walking the castle grounds gives a sense of the original scale of the fortification. The area around the castle was the centre of Mito's Confucian intellectual tradition, and the Shokokan archive building — where the great Mito history project Dai Nihon-shi was compiled over two and a half centuries — stood nearby.